With respect to permanent magnet motors, a trade off exists between permanent magnet motors having high torque at low speed and between permanent magnet motors having a wide speed range. If the permanent magnet motor is built such that large amounts of low end torque are produced, then the top end speed is reduced. If a high top speed is required, then the permanent magnet motor is built at the expense of low end torque.
For use in electrically-powered automobiles, it is desirable to have both high torque at low speed and a high top speed of the motor to avoid the necessity for a transmission assembly (which increases cost and complexity) to increase the speed range of the permanent magnet motor and still provide enough torque to accelerate the automobile at an acceptable rate. Several methods presently exist to address these concerns. These current methods reduce the flux density in the stator core. These methods typically include the operations of: (1) varying the air gap between the rotor and stator to alter the flux density in the stator and (2) introducing currents into the stator that create magnetic fields that oppose the magnetic fields of the permanent magnets on the stator.
Varying the air gap often requires very complex mechanical assemblies that involve frictional sliding between mechanical components as the rotor moves axially relative to the stator. Under torque load, however, frictional slide can be problematic. For example, frictional sliding is often associated with the “stick-and-slip” phenomena, imposing a great challenge for controlling the desired air gap.
It would be advantageous to provide a mechanism for axially adjusting the rotor assembly which is not restricted by frictional sliding between mechanical components, and accordingly, which may be easily adjusted under torque load conditions to accommodate a wide range of torque and speed settings for the motor.